Insulated wires coated with an electrical insulator are used in a large amount in many applications as coils incorporated into a variety of electric machinery and tools. Many insulated wires are used especially in electric machinery and tools, represented by motors and transformers. In recent years, progress has been made in both miniaturization and the level of performance characteristics of these machinery and tools, and now such insulated wires are often used by bundling them into a very narrow space. Specifically, it is no exaggeration to say that the performance of a rotator, such as a motor, is determined by how many wires are held in a cross section of a stator slot. As a result of this, the ratio (space factor) of the sectional area of a conductor to the sectional area of the stator slot has been highly increased in recent years.
When insulated wires whose cross section has a round shape are closely packed inside of a stator slot, there are problems about voids forming dead space and the sectional area of an insulating film. For avoiding this, in order to improve the space factor as much as possible, users push the wires into the stator slot to such an extent that the wires having a cross section of a round shape are deformed. However, as expected, remarkable reduction in the sectional area of the insulating film has not been made, because it may sacrifice electrical performance (e.g. dielectric breakdown).
From the result, as means for improving the space factor, an attempt has been made, very recently, to use a rectangular wire with a conductor having a sectional shape similar to a quadrangle (a regular square or a rectangle). Use of the rectangular wire brings about a dramatic improvement in the space factor. However, it is difficult to apply an insulating film uniformly on a rectangular conductor, and it is particularly difficult to control the thickness of the insulating film in the case of insulated wire having a small sectional area. Therefore, the rectangular wire has not been widely used so much.
Various methods are proposed to provide the wire film with winding processing property. These methods include one in which lubricity is imparted to a film, to lower the friction coefficient, thereby decreasing external damage during coiling, and a method in which adhesion between a film and an electric conductor is improved, to prevent the film from peeling from the conductor, thereby maintaining the electrical insulating property.
As the method of imparting lubricating ability, a method in which the surface of a wire is coated with a lubricant, such as wax, or a method in which a lubricant is added in an insulating film and then is allowed to breed out on the wire surface when a wire is produced, thereby imparting lubricating ability, is conventionally adopted and there are many examples.
First, among these means that have been hitherto used, as the method of reducing the friction coefficient of the surface of an insulating film, it is proposed that a wax, an oil, a surfactant, a solid lubricant, or the like is coated on the surface of an insulated wire. It is also known that an anti-friction agent made of a wax that can be emulsified in water and a resin that can be emulsified in water and solidified by heating, is coated and baked, and then the resultant is used. Furthermore, suggested are a method of adding polyethylene fine powder to an insulating coating itself, thereby attaining lubrication, a method of blending a wax component and a crosslinking agent with an insulating coating, and the like.
The above-mentioned methods have been contrived to improve the surface lubricity of an insulated wire so as to improve the winding processing property by the surface slipping of the wire.